1. Field of the Invention
The invention relates to an image enhancing apparatus which varies frequency characteristics of a detail signal in accordance with a level of a luminance signal and a video camera having such an image enhancing apparatus.
2. Description of the Related Art
As an image enhancing apparatus which is used in a signal processing circuit of a video camera, an apparatus with a construction as shown in FIG. 1 has conventionally been used. In FIG. 1, delay outputs of 0H (this means that a delay amount is equal to 0), 1H, and 2H of a red signal R are respectively supplied to an HPF (high pass filter) 21a and an LPF (low pass filter) 21b in the vertical direction. Delay outputs of 0H, 1H, and 2H of a green signal G are respectively supplied to an HPF 22a and an LPF 22b in the vertical direction. Further, delay outputs of 0H, 1H, and 2H of a blue signal B are respectively supplied to an HPF 23a and an LPF 23b in the vertical direction.
The HPFs 21a, 22a, and 23a form high frequency signals in the vertical direction of chrominance signals. Output signals of the HPFs 21a, 22a, and 23a are mixed by a mixer 24a at a predetermined mixture ratio. A vertical high band signal VH from the mixer 24a is transmitted through an LPF 25 in the horizontal direction, so that vertical high band.cndot.horizontal low band signal VHHL is formed and supplied to a mixer 30.
The LPFs 21b, 22b, and 23b in the vertical direction form low frequency signals in the vertical direction of the chrominance signals. Output signals of the LPFs 21b, 22b, and 23b are mixed by a mixer 24b at a predetermined ratio. A vertical low band signal VL from the mixer 24b is transmitted through a BPF (band pass filter) 26 in the horizontal direction, so that a vertical low band.cndot.horizontal high band signal VLHH is formed. The BPF 26 is constructed so that a center frequency (ordinarily, about 2 to 8 MHz) can be varied by a manual adjustment in accordance with a taste of the user or a feature of an image which was photographed. The signal VLHH is supplied to the mixer 30. The mixer 30 adds the vertical high band.cndot.horizontal low band signal VHHL and vertical low band.cndot.horizontal high band signal VLHH. The added output signal is supplied to a multiplier 27.
The vertical low band signal VL from the mixer 24b is supplied to an LPF 28 in the horizontal direction. A vertical low band.cndot.horizontal low band (DC components) VLHL is formed by the LPF 28. The signal VLHL is supplied to a non-linear circuit 29. For example, the non-linear circuit 29 has a comparator and has non-linear input-output characteristics and executes a non-linear process so that a level near a black level of an input signal is set to 0 and a level that is equal to or higher than a predetermined level is set to 1. Such a process is "level dependent". An output signal of the non-linear circuit 29 is supplied to the multiplier 27. A gain of an output signal of the mixer 30 is controlled by the multiplier 27. A detail signal DTL is taken out from an output terminal 31.
Since a 1H delay line has a large circuit scale and is expensive, it is also possible to use such a delay line for only the G channel or for only the G and R channels and to form the vertical high band.cndot.horizontal low band signal VHHL and vertical low band.cndot.horizontal high band signal VLHH. In such a case, the LPF in the vertical direction is not provided for a channel without the 1H delay line but the signals are directly supplied to the mixers 24a and 24b for details in the horizontal direction. In FIG. 1, blocks 21a, 21b, 23a, and 23b surrounded by broken lines show circuit blocks which can be omitted in case of using only the G channel.
In the conventional image enhancing apparatus as mentioned above, a gain of the detail signal near the black level is reduced, thereby preventing a deterioration of an S/N ratio. However, in a digital signal processing camera, a problem such that when noises near the black level are removed, the details are deleted and when the details are not deleted, the noises become conspicuous occurs.
In a video camera, a gamma correcting circuit is ordinarily provided. According to input/output characteristics of the gamma correcting circuit, since a gain is large for an input at a low level, as for an image pickup signal, a video level of the input near the black level is amplified to about four times by a gamma correction. The same shall also similarly apply to a case where the detail signal formed by the image enhancing apparatus passes through such a gamma correcting circuit. Thus, the S/N ratio deteriorates and noises near the black level are further conspicuous for the human eyes.
In the conventional image enhancing apparatus mentioned above, therefore, the DC components transmitted through the non-linear circuit 29 are supplied to the multiplier 27 and the gain of the detail signal is controlled, thereby reducing a gain of the details near the black level. Such a process is called a "level dependent". In an analog circuit, generally, since the product of G (gain) and B (band width) is constant, when the gain of the gamma correcting circuit near the black level rises, the band width is narrowed and the high frequency components also naturally decrease. Therefore, there is no need to largely make the "level dependent" effective.
Different from the analog circuit, however, in the digital circuit, even if the gain near the black level is raised by the gamma correcting circuit, frequency characteristics are not changed. Namely, the high frequency components don't decrease, high frequency noises are increased by only an amount corresponding to the increased gain, and the "level dependent" has to be strongly made effective. However, when the "level dependent" is excessively made effective, the detail signal near the black level is hardly extinguished and an image becomes very unnatural. For example, when a bright image and a dark image exist, a phenomenon such that the detail signal is added to only the bright image occurs.